Identification of the major in vivo ABl-42-degrading catabolic-system in brain parenchyma.

淀粉样蛋白-p-β肽(AP)在大脑中的积聚触发了导致阿尔茨海默病的病理级联反应，从而导致阿尔茨海默病。为了探讨AB分解代谢的机制，我们分析了多重放射性标记的~3H/14C-AβL-42注入大鼠海马区的去向，并用一组肽酶抑制剂鉴定了与Neprilysin相似或相同的中性内肽酶(S)作为限速肽酶。向大鼠海马区长期注入奈普利辛的特异性抑制剂硫吗芬可引起内源性AB的蓄积。为了进一步鉴定硫吗啡敏感的多肽酶，我们分析了中性内肽酶家族中的六种重组硫吗啡敏感多肽酶对ABL-42的分解代谢。其中，奈普利辛对AB的降解最快、最有效。奈普赖素缺乏的小鼠显示出分解AB的能力显著降低。我们还观察到了小鼠脑内内源性AB水平(-/-&gt；+/-&gt；+/+)随基因剂量的变化。Neprilysin缺陷小鼠大脑中AB42水平的增加与携带PSI突变的小鼠相当。此外，野生型小鼠脑内的区域AP水平由高到低依次为：海马、皮质、纹状体/丘脑、小脑，这一趋势在neprilysin缺陷小鼠中明显夸大。在用老龄小鼠进行的实验中，30个月龄的小鼠海马区Neprilysin活性及其蛋白质含量比2个月龄的小鼠降低了200%。随着年龄的增长，大脑皮层的Neprilysin活性也略有下降，但显著降低，而其他区域的活性没有变化。这些观察结果表明，尽管neprilysin家族存在分子冗余，但neprilysin仍是脑内主要的AB降解酶，并且neprilysin活性的降低，特别是在易发生AB病理的区域，将通过促进AB沉积而促进阿尔茨海默病S病的发生。

Accumulation of amyloid-pβ peptide (Ap) in brain triggers the pathological cascade leading to Alzheimer' s disease. To examine the mechanism of AB catabolism, we analysed the fate of multiply radio-labeled 3H/14C-Aβl-42 injected into rat hippocampus and identified neutral endopeptidase (s) similar or identical to neprilysin as the rate-1imiting peptidase using a panel of peptidase inhibitors. Chronic infusion of thiorphan, a specific inhibitor of neprilysin, into rat hippocampus caused accumulation of endogenous AB. To further identify the thiorphan-sensitive peptidase, we analysed ABl-42 catabolism by six recombinant thiorphan-sensitive peptidases in neutral endopeptidase family. Neprilysin degraded AB most rapidly and efficiently among them. Neprilysin-deficient mice showed significantly reduced ability to catabolize AB. We also observed a gene dosage-dependent clevation of the endogenous AB levels (-/- > +/- > +/+) in the mouse brains. The increase in the AB42 level in the neprilysin-deficient mouse brain was comparable to that in the mice carrying PSI mutation. Moreover, the regional Ap levels in wild-type mouse brain were in the order of hippocampus > cortex > striatum/thalamus > cerebellum and this tendency was markedly exaggerated in the neprilysin-deficient mice. In the experiments using aged mice, neprilysin activity and its protein content in hippocampus were reduced by 200% at 30 months-old ages, compared to 2 months-old ages. The neprilysin activity in cortex was also slightly but significantly reduced with aging, whereas those in other regions were not changed. These observations suggest that the neprilysin is the major AB-degrading enzyme in brain despite the molecular redundancy of the neprilysin family and that reduction of neprilysin activity particularly in the regions vulnerable to AB pathology will contribute to Alzheimer' s disease development by promoting AB deposition.